1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method which require that a developing bias is applied upon a toner carrier with an image carrier seating an electrostatic latent image positioned facing the toner carrier which carries toner so that the toner accordingly moves to the image carrier from the toner carrier and the electrostatic latent image is visualized.
2. Description of the Related Art
Known as image forming apparatuses, such as copier machines, printers and facsimile machines, to which electrophotographic techniques are applied are two types: those apparatuses of the contact developing type according to which an image carrier and a toner carrier are held abutting on each other; and those apparatuses of the non-contact developing type according to which an image carrier and a toner carrier are held away from each other. Of these, in an image forming apparatus of the contact developing type, a toner carrier is applied a developing bias with a direct current voltage or a voltage which is obtained by superimposing an alternating current voltage upon a direct current voltage, and when toner carried by a surface of the toner carrier contacts an electrostatic latent image which is formed on an image carrier, the toner partially moves toward the image carrier in accordance with a surface potential of the electrostatic latent image, and a toner image is consequently formed.
Meanwhile, in an image forming apparatus of the non-contact developing type, an alternating voltage serving as a developing bias is applied upon a toner carrier, an alternating field develops in a gap between the toner carrier and an image carrier, toner transfers owing to the function of the alternating field, and a toner image is consequently formed.
In an apparatus with electrophotographic techniques, an image density of a toner image may vary because of an individual difference which the apparatus has, a change with time, a change in environment surrounding the apparatus such as a temperature and humidity, etc. Noting this, various types of techniques have been proposed which aim at image density stabilization. Such techniques include one which requires to form a small test image (patch image) on an image carrier to thereby optimize a density control factor which influences an image density based on a density of the patch image. According to this technique, predetermined patch images are formed on an image carrier while changing a density control factor, a density sensor disposed in the vicinity of the image carrier detects an image density of the patch images and the density control factor is adjusted such that the density will match with a predetermined target density, in an effort to obtain a desired image density.
For example, according to an image density control technique described in Japanese Patent Application Laid-Open Gazette No. 2002-72584, (1) when a power source of an apparatus main unit is ON, (2) at the time of exchanging a process cartridge or a developer cartridge, (3) upon receipt of a new print instruction in a condition that the apparatus has not long been used, and (4) when a predetermined number of pages have been printed, a predetermined toner patch is formed prior to formation of the next image, a developing bias serving as a density control factor is changed based on a density of the toner patch, and an image density is accordingly controlled.
With respect to an apparatus of this type, it is known that when a power source is OFF or when an operation-suspended state that image formation is not executed has been continuing for long time even through the power source is ON, an image formed through a subsequent image forming operation may show cyclic density variations. While such density variations are gradually eliminated as the image forming operation is repeated a few times, if the operation-suspended state lasts for long, a longer period of time will be necessary to eliminate density variations and an image quality may deteriorate even to a measurable extent in some cases.
In an image forming apparatus according to a conventional technique in particular which requires to form a patch image for adjustment of a density control factor, when a patch image is formed after such an operation-suspended state, a density variation as that described above may lead to a variation in density of the patch image. This causes a problem that it is difficult to accurately adjust the density control factor based on a density of the patch image and it therefore is difficult to form a stable image.